are relays necessary

Specifically...... Often times, car alarms ALREADY have built in relays.   If the built in relay is rated high enough for the job, would you STILL use an external relay? It seems that the internal relays in most car alarms are much smaller than full size external relays and maybe its that smaller size that equates to less ability to handle current?

If there is a built in relay to handle a certain function such as lock variations, use it. You only need relays when you need to flip the polarity and or need more current.

I almost never use external relays if the alarm has one onboard. However, there are exceptions. Even the units that have an onboard trunk-release relay often don't give you access to terminal 87a, so you can't use the internal relay for a "5-wire" trunk like on a Camry or new Corolla. If you're doing remote start on a car that has a large amount of parking lights, like, say, a pickup with roof clearance lights, fog lights that come on with the parking lights, a connected trailer with even more lights.....might not be a bad idea to use an external relay for the parking lights. Also, relays (onboard or outboard) make clicking sounds, which could give a car thief a clue as to where they're mounted. He could follow the sounds, and find where the alarm is installed to disable or unplug it. If you're trying to do a super-stealth security install with everything hidden, you might consider using outboard relays, and mount them somewhere separate from the alarm brain.

Thanks for the assists guys ! One other RELAY question.... On the common 4 terminal Bosch 12v automotive relay, what is the limit to the current you can apply across the coil? The LOAD side rating is clearly stated on the package (ie, 10A,20A,30A etc, but I can't find anything about the limits to the current you can supply to the coil side? Thanks

It doesn't matter in this case as it is only running the coil. Anything from a MA output to 50amps, as long as it is 12volts.Mike M2

Tech Manager

CS Dealer Services